Leak detector for vaporization cooled transformers

ABSTRACT

A vaporization cooled transformer of the type comprising a transformer tank in combination with a heat exchanger employs temperature sensing means in both the tank and the heat exchanger for sensing a predetermined temperature differential to indicate the presence of air in the transformer. When the temperature differential exceeds a predetermined range, an alarm and a transformer disconnect relay become actuated. The temperature differential sensor can also be calibrated to provide excess moisture indication above a threshold moisture content caused by the saturation of the molecular sieve water scavenger within the transformer assembly.

This is a division of application Ser. No. 021,543, filed Mar. 19, 1979,now U.S. Pat. No. 4,232,551.

BACKGROUND OF THE INVENTION

U.S. patent application Ser. No. 843,676 filed Oct. 19, 1977, nowabandoned discloses a vaporization cooled transformer wherein avaporizable fluid is used for providing both cooling facility anddielectric capability to transformer cores and windings. An effectiveliquid level gage for sensing the quantity of coolant is described inU.S. patent application Ser. No. 939,123 filed Sept. 5, 1978, nowabandoned. The liquid level gage adequately provides shutoff facilitywhen a vaporizable fluid such as trichlorotrifluoroethane leaks out ofthe transformer tank. The aforementioned vaporization cooled transformerutilizes a quantity of molecular sieve material in the vapor pathbetween the transformer tank and the heat exchanger as a water scavengerto remove any moisture that may be released from the cellulosicinsulation materials during transformer operation. Since the insulatingmaterial continuously release water vapor to the transformer interiorover the operating life of the transformer, a sufficient quantity of themolecular sieve material is employed to provide for adequate wateradsorption throughout the operating life of the transformer. In theevent that the sieve material becomes saturated, excess moisture canoccur within the transformer and behave as an ideal gas under certaintemperature conditions. The excess moisture under these conditions cancause corrosion of the heat exchanger.

When leaks develop through openings occurring within the heat exchangeror tank assembly a negative internal pressure within the heat exchangeror tank assembly allows a substantial quantity of ambient air to enterthrough the leak openings. The presence of a quantity of atmospheric airwithin the heat exchanger can be detrimental to the transformeroperation. One long-term deleterious effect is the premature saturationof the molecular sieve material due to the presence of substantialquantities of water vapor present within the admitted air.

Short-term deleterious effects which can occur due to the presence ofthe admitted air include both an overpressure condition caused byreduced heat exchanger efficiency as well as coolant loss by the escapeof the vaporizable coolant out through the leak openings. The heatexchanger efficiency is decreased because the presence of the trappedair within the heat exchanger headers and cooling tubes prevents thevaporized coolant from entering into these areas during the condensationperiods of the vaporization-condensation cycle. The loss in coolingefficiency in turn causes the transformer to operate at a highertemperature causing further increases in pressure until an overpressuremechanism becomes energized and the transformer becomes automaticallydisconnected.

The purpose of this invention is to provide means for sensing thedifferences in temperature that exist between the heat exchanger and thetransformer tank to determine the presence of admitted air within theheat exchanger assembly as well as the presence of excess moisture.

SUMMARY OF THE INVENTION

Temperature sensing means are installed both in the heat exchanger andthe transformer tank in vaporization coold transformers. The temperaturedifferential between the heat exchanger and the tank is sensed todetermine when a predetermined temperature differential is exceeded. Theexcess temperature differential indicates the presence of ambient air toexcess moisture within the transformer. Alarm indicating means andtransformer disconnect relays are actuated when the temperaturedifferential exceeds the predetermined range.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graphic representation of the coolant vapor pressure of avaporization cooled transformer as a function of transformer loading forcontours of ambient air temperature;

FIG. 2 is a front section view of a vaporization cooled transformercontaining the leak detection means according to the invention;

FIG. 3 is a top section view of the heat exchanger of FIG. 2 through theplane 3--3;

FIG. 4 is an enlarged section view of the lower header within theembodiment of FIG. 1 with the temperature sensor connected to the end ofthe header;

FIG. 5 is a side section view of a vertically arranged heat exchanger asan alternative to the horizontally arranged heat exchanger of FIG. 2;

FIG. 6 is a graphic representation of the relation between temperatureand time for the embodiment of FIG. 2 in the absence of a leak;

FIG. 7 is a graphic representation of the relation between temperatureand time for the embodiment of FIG. 2 in the presence of a leak;

FIG. 8 is a graphic representation of the temperature differentialbetween the temperature depicted in FIGS. 6 and 7 as a function of time;

FIGS. 9A-9C are diagrammatic representations of the temperatureseparation which occurs within the side, front and bottom section viewsof a portion of a heat exchanger at 25% transformer loading;

FIGS. 10A-10C are diagrammatic representations of the temperatureseparation which occurs within the side, front and bottom section viewsof a portion of a heat exchanger at 75% transformer loading;

FIGS. 11A-11C are diagrammatic representations of the temperatureseparation which occurs within the side, front and bottom sections of aportion of a heat exchanger at 100% transformer loading;

FIG. 12 is a front section view of a vaporization cooled transformercontaining a further arrangement of the leak detection means of theinvention;

FIG. 13 is a graphic representation of the relation between temperatureand time for the embodiment of FIG. 12 in the absence of a leak;

FIG. 14 is a graphic representation of the relation between temperatureand time for the embodiment of FIG. 12 in the presence of a leak;

FIG. 15 is a graphic representation of the temperature differentialbetween the temperatures depicted in FIGS. 13 and 14 as a function oftime; and

FIG. 16 is a diagrammatic representation of the temperature separationwhich occurs within a vaporization cooled transformer depicted in apartial section front view having vertical cooling tubes and containingthe leak detection means according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows the relationship between vaporized coolant pressure andtransformer loading existing within a vaporization cooled transformerfor various ambient temperatures. It can be seen that the coolant vaporpressure within the transformer at low conditions of loading and lowambient temperatures can be quite low. A standard atmospheric pressureline P is included at 15 pounds per square inch absolute (PSIA) forcomparison purposes. Pressures beneath the standard atmospheric pressureline P are considered as negative pressures for the purposes of thisdisclosure.

FIG. 2 contains a vaporization cooled transformer 10 as described withinthe aforementioned U.S. Patent applications which are incorporatedherein by way of reference. The vaporization cooled transformer 10comprises a transformer tank 11 and a heat exchanger 12 wherein thetransformer windings 13, core 14, and bushing 15 are cooled by means ofvaporizable coolant 16. The coolant becomes vaporized by the heatgenerated by the transformer core and the windings and the vaporizedcoolant 16' enters up into the intake manifold 17 within the heatexchanger by means of intake pipe 18 in the direction indicated by arrowB. A plurality of headers 19 are connected to the intake manifold and inturn are connected with a plurality of inclined cooling tubes 20connecting between the intake manifold 17 and return manifold 21. Thecondensed coolant droplets 16" return to the transformer tank from thereturn manifold by means of return pipe 22. An expansion tank 23 isprovided for the expansion of the vaporized coolant and is connectedwith the intake manifold by means of connector pipe 24. Any coolantvaporizing within the expansion tank returns to the transformer tank bymeans of expansion return pipe 25. Located between the transformer tank11 and intake pipe 18 is a molecular sieve container 26 consisting of aquantity of moisture adsorbing molecular sieve material 27 withinhousing 28 which contains a plurality of apertures 29 for retaining themolecular sieve material while permitting the transport of the coolant.Upon becoming vaporized, coolant 16' transmits out from the transformertank through opening 30 in the direction indicated by arrow A.

As previously discussed, the presence of a leak in any portion of theheat exchanger 12 or tank 11 causes an influx of ambient air into theheat exchanger assembly. During transformer operation the admitted airstabilizes within heat exchanger headers 19 after a period of extendeduse and settles in lowest header 19' within the intake manifold. Thepresence of a quantity of admitted air within any of the headers 19prevents vaporized coolant 16' from entering those headers and theirassociated cooling tubes 20 so that the headers and cooling tubes remainat a somewhat lower temperature than the remaining cooling tubes andheaders. This is because the vaporized coolant is prevented fromcondensing and transmitting its heat of vaporization within the "airlocked" headers and cooling tubes. The air locked condition also occurswhen fans are employed to increase the cooling efficiency of the heatexchanger. The insertion of a temperature sensor 31 such as athermocouple, within the lowest header 19' and a sensor 32 within thetransformer tank provides one means for sensing the temperature of thelowest header 19' and of the coolant within the transformer tank at anygiven time. Connecting the thermocouples 31, 32 by means of electricalconnectors 33, 34 to a differential temperature gage 35 allows thetemperature difference between the header and the transformer tank to berapidly determined. Once the temperature differential is determined adirect readout is provided by indicator 36 in order for an operator tomonitor the temperature differential while the transformer is operating.A terminal 37 is provided on the differential temperature gage in orderto provide an output signal for connecting to an audio-visual alarmdevice and to a circuit breaker relay for interrupting input power tothe transformer when a predetermined temperature differential isexceeded.

The temperature sensor 31 can be connected to heat exchanger 12, whichis horizontally arranged, by connection through the side of the lowestheader 19' as indicated in FIG. 3 or by connection through the end ofthe lowest header 19' as shown in FIG. 4.

The temperature sensor 31 can be located at the bottom of heat exchanger12 of FIG. 5 by connecting through the side of lowest header 19' whichis higher than entrance pipe 18 as indicated.

For normal operating conditions and in the absence of any leaks thetemperature of the interior surface of the header or tube beingmonitored is approximately the same temperature as the temperature ofthe vaporized coolant 16' within the associated header or tube. A seriesof tests were performed on a 1000 Kva transformer within a 31° C.ambient. FIG. 6 shows the temperatures from time of startup at arelatively high load value of 15,380 watts. As shown, the temperaturesof the coolant in the tank A and the coolant vapor in the lowest headerB, are approximately equal. The temperature of the lowest header isabout 3° C. less due to thermocouple errors and conduction heat lossthrough the header. It had prevously been concluded that upontransformer startup, coolant in the tank would heat up in the vicinityof sensor 32 (FIG. 2) before hot coolant vapor could move through theheat exchanger to sensor 31. However, the coolant vapor rapidly reachessensor 32 so that sensor 32 stays approximately at the same temperatureas sensor 31 during the startup cycle. This is indicated by the nearequal temperature conditions for both A and B.

A quantity of air admitted to the transformer resulted in thetemperatures A and B shown in FIG. 7. When the air was admitted into thetransformer, the load had to be reduced to 9770 watts in order not todamage the windings. For this load value the steady state pressuremeasured 8.6 psig. For the temperature values shown in FIG. 6 thepressure measured 5.3 psig at the higher load value. As the transformercontaining the admitted air is further loaded, the temperature of sensor32 begins to rise. However, due to the air leak, the temperature ofsensor 31 remains constant for the first 30 minutes. The temperaturedifference ΔT₀ is 6.7° C. after 15 minutes. The temperature differenceΔT₁ is 18.8° C. after one hour. The leak detector can therefore give anearly indication of the presence of a leak before a dangerous overpressure develops. The temperature of sensor 31 begins to heat up afterone hour, possibly due to the hot vapor in the heat exchangerintermixing with the admitted air. However, the lowest header remains ata lower temperature due to the air leak. At steady state conditions thetemperature difference ΔT₂ was still approximately 13.4° C.

FIG. 8 is a graphic representation of the temperature differentialoccurring between sensors 31 and 32 with and without a leak. An alarmcan be connected to terminal 37 on the temperature gage 35 in FIG. 2 andpreset above approximately 6° C. to allow for sensor errors and loaddeviation. Temperature differentials above preset values of from 0° to10° C. would operate the alarm to give an audible and visual indicationof the presence of a leak.

The leak detector which comprises the combination of temperature gage 35and sensors 31 and 32 can have a direct reading visual scale, an audiblealarm, or a visual and audible signaling mechanism, depending ontransformer location and operator preference. The leak detector can alsobe electrically coupled with a transformer disconnector relay.

FIGS. 9 through 11 depict the location of the hot vapor and cooler airin a 750 Kva transformer for loads of 3600, 9000, and 12,000 watts. The12,000 watts value represents the approximate full load rating and the9000 watt and 3600 watt values represent 75% and 25% load valuesrespectively. As shown in FIGS. 9A-9C for 25% loading and FIGS. 10A-10Cfor 75% loading, the lowest row of cooling tubes 20' are partially coldat 25% loading and entirely hot at the higher 75% rating. When theloading is increased to 100% the lowest row of cooling tubes has a welldefined cool region as shown in FIGS. 11A-11C.

When loads are less than 100%, the increase in coolant vapor pressure,which occurs in the presence of a leak, is not as detrimental as fullload conditions and can be withstood until the load conditions increasefurther to 100%.

As shown in FIG. 12, sensor 31 may be located in the highest header 19".An additional sensor 31' can be located within expansion tank 23 andconnected to gage 35 by conductor 38 for use in combination with sensor31 so that an indication can be received from either sensor 31 or sensor31' relative to sensor 32.

Under leak conditions at full load, temperature increases were sensed atsensor 32 while the temperatures at 31 and 31' decreased to lowerlevels. Temperature differentials in excess of the 6° C. setting aresufficient to cause standard overpressure gages to become energized athigh loads and to cause the transformers to be automatically shut downby standard auxiliary relay equipment. This occurs since pressuresgenerated in excess of the standard overpressure gage setting of 15 psigcan cause damage to the heat exchanger assembly. It is to be noted thatpressure increases within the heat exchanger assembly and thetransformer tank during operation cause a corresponding increase in theboiling temperature of the coolant so that the transformer operates at ahigher temperature.

The temperatures monitored at sensor 32 within the transformer tank andsensor 31' within the expansion tank, are shown at C and D respectivelyin FIG. 13 for transformer operation in the absence of a leak. The sametemperatures monitored, when air was admitted are shown in FIG. 14. Itis to be noted that a differential occurs similar to that describedearlier for the header mounted sensor 31 of FIG. 2. This differential isshown in FIG. 15 under both regular and leak conditions over a period oftime.

Intentionally forming leaks of various sizes at different locationswithin the horizontal heat exchanger assemblies of FIGS. 2 and 12confirmed the fact that the admitted air generally settles in the lowestheader on the intake manifold side of the heat exchanger assembly. Thereason for the settling of the admitted air in the lowest header withinthe intake manifold is not at this time well understood. It is notedhowever, that regardless of where the leak occurs most of the admittedair does in fact locate within the lowest header within the intakemanifold. There is also a temperature differential occurring within thelowest cooling tubes 20' associated with the lowest header 19' relativeto the temperature of the coolant within the transformer tank.

The leak detector of this invention can be used with other heatexchanger designs. FIG. 16 depicts a vertical type heat exchanger 12described in aforementioned U.S. patent application Ser. No. 843,676filed Oct. 19, 1977. Air leaking into this heat exchanger 12 consistingof vertical tubes 20, segregates in upper header 40' as well as withinthe upper region of expansion chamber 41 and cooling tubes 20. Sensor 31may optionally be located within upper header 40' or within expansiontank 41.

Since the remainder of the cooling tubes and headers are hot relative tothe top end portions of the tubes and headers it is to be clearlyunderstood that sensor 32 can be connected to the hot portions of theheaders and cooling tubes as well as to the transformer tank. Anauxiliary sensor 32' connected to the hot portion of one of the headers40 can provide a temperature differential relative to sensor 31 in thecolder region of the heat exchanger.

Sensor 32 may be mounted within coolant 16 in tank 11, in the hot vaporspace above the coolant as shown in FIGS. 2 and 12, or within the mainvapor supply pipe 18 to the heat exchanger. Sensor 32 can also belocated within the hot portion of one of the headers 40 or cooling tubes20 in the heat exchanger for providing a temperature differentialrelative to sensor 31.

The leak detector of the invention also provides effective high moistureindication. When the molecular sieve material 27 in FIG. 2 becomesinoperative due to excessive moisture within the transformer, the excesswater vapor behaves as a noncondensable gas and segregates in a mannersimilar to the admitted air in the presence of a leak and provides thesame hot and cold regions indicated earlier in FIGS. 9-11.

Although thermocouples are used as the temperature sensors within thetransformer tank and the heat exchanger assembly, other temperaturesensing means such as thermistors, resistive elements and direct readingthermometers can also be employed.

It is to be understood that the pressure within both the heat exchangerand the transformer tank can increase for a variety of reasons. Thisinvention is directed to means for determining the presence of a leakeither within the heat exchanger assembly or the transformer tank abovethe coolant liquid level that is not readily detectable until thetransformer becomes energized. The presence of admitted air within thetransformer tank, for example, can cause serious dielectric problemsimmediately upon transformer startup. The admitted air is notdeterminable with standard type pressure sensing means either within thetank or heat exchanger assembly. A sufficient quantity of admitted airwill immediately indicate a temperature differential between sensor 31located within the heat exchanger and sensor 32 within the transformertank before any dielectric breakdown problems can occur within thetransformer windings during startup conditions.

Although the leak detector of the invention is directed to vaporizationcooled transformers, this is by way of example only. The invention findsapplication wherever heat exchangers and vaporization cooledelectrically heated elements may be employed.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:
 1. An excess moisture detector for vaporization cooledtransformers comprising:a transformer tank containing a condensablecoolant and having an opening in the top; a heat exchanger assemblyconnected to the top of said transformer tank for receiving said coolantin vapor form and returning said coolant in condensed form to the tank;a quantity of moisture adsorbing material in an apertured containerlocated intermediate the heat exchanger and the tank for removingmoisture from the coolant, said moisture adsorbing material beingmoisture saturated due to excessive moisture within said transformer;temperature sensing means within a portion of the heat exchanger todetermine the temperature within said heat exchanger; temperaturesensing means within the tank to determine the temperature of thecoolant within the tank; and indicating means for determining thedifferences in the heat exchanger temperature and the coolanttemperature and for providing an indication when a predeterminedtemperature differential is exceeded thereby detecting that saidmoisture adsorbing material has become inoperative due to the presenceof excess moisture.